Field of the Invention
The present invention relates to a circuit configuration for the offset compensation of a signal, particularly of a frequency-modulated received signal in a cordless digital communication system. The invention also relates to a method for offset compensation by using such a circuit arrangement in order to be able to compensate for low-frequency or direct-current offset components contained in the respective signal.
In many cordless digital communication systems such as, for example, communication systems according to the DECT (digital enhanced cordless telecommunications), WDCT (world-wide digital cordless communications), Bluetooth, or the SWAP (shared wireless access protocol) mobile radio standard, FM (frequency modulation) demodulators are used for receiving frequency-modulated radio-frequency signals. In these systems, a GFSK (Gaussian Frequency Shift Keying) modulation is frequently used as a type of digital frequency modulation. As a rule, the output signal of such FM demodulators is disturbed by an offset that changes slowly in a time-dependent manner and is caused by equipment tolerances, frequency offsets or drift between the transmitter and the receiver. This offset is not known in advance.
For this reason, the use of circuit arrangements for compensating for such offsets is required in the receivers so that the symbols of the offset-compensated received signals can then be decided or assessed in an undisturbed and correct manner as a result of which the performance of the receiver can be optimized and the bit error rate can be reduced for a predetermined signal/noise ratio.
In conventional analog receivers, analog circuits are exclusively used for compensating for a low-frequency or direct-current offset. These known analog circuits consist of an analog low-pass filter for approximately estimating the offset signal and then subtracting the low-pass-filtered signal from the input signal in dependence on this estimate which, overall, corresponds to high-pass filtering.
However, this exclusively analog principle is not suitable if relatively short time intervals such as, for example, the preamble of a digital bluetooth communication signal with a duration of only 4 μs have to be compensated for or filtered, respectively.
In International Publication WO 97/27695 A, a receiver with a recursive digital filter is described. The filter is used for eliminating phase and frequency offsets from a received signal. The filter has time-variable filter coefficients for this purpose.